Implantable stimulation devices generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. patent application Ser. No. 11/177,503, filed Jul. 8, 2005, which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. An SCS system typically includes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitter and receiver, electrodes, at least one electrode lead, and, optionally, at least one electrode lead extension. The electrodes, which reside on a distal end of the electrode lead, are typically implanted along the dura of the spinal cord, and the IPG or RF transmitter generates electrical pulses that are delivered through the electrodes to the nerve fibers within the spinal column. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing to create an electrode array. Individual wires within one or more electrode leads connect with each electrode in the array. The electrode lead(s) exit the spinal column and generally attach to one or more electrode lead extensions. The electrode lead extensions, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG or RF transceiver is implanted. Alternatively, the electrode lead may directly connect with the IPG or RF transceiver. For examples of other SCS systems and other stimulation systems, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties. Of course, implantable pulse generators are active devices requiring energy for operation, such as is provided by an implanted battery or an external power source.
As should be obvious, an IPG needs electrical power to function. Such power can be provided in several different ways, such as through the use of a rechargeable or non-rechargeable battery or through electromagnetic (EM) induction provided from an external charger, or from combinations of these and other approaches, which are discussed in further detail in U.S. Pat. No. 6,553,263 (“the '263 patent”), which is incorporated herein by reference in its entirety. Perhaps the favorite of these approaches is to use a rechargeable battery in the IPG, such as a lithium-ion battery or a lithium-ion polymer battery. Such a rechargeable battery can generally supply sufficient power to run an IPG for a sufficient period (e.g., a day or more) between recharging. Recharging can occur through the use of EM induction, in which EM fields are sent by an external charger to the IPG. Thus, when the battery needs recharging, the patient in which the IPG is implanted can activate the external charger to percutaneously (i.e., through the patient's flesh) charge the battery (e.g., at night when the patient is sleeping or during other convenient periods).
The basics of such a system are shown in FIG. 1, which is largely descriptive of salient contents of the '263 patent. As shown, the system comprises in relevant part the external charger 208 and IPG 100. As noted, a coil 279 in the charger 208 produces an EM field 290 capable of percutaneous transmission through a patient's flesh 278. The external charger 208 may be powered by any known means, such as via a battery or by plugging into a wall outlet, for example. The EM field 290 is met at the IPG 100 by another coil 270, and accordingly an AC voltage is induced in that coil 270. This AC voltage is turn is rectified to a DC voltage at a rectifier 682, which may comprise a standard bridge circuit. (There may additionally be data telemetry associated with the EM field 290, but this detail is ignored as impertinent to the present disclosure). The rectified DC voltage is in turn sent to a charge controller 684, which operates generally to regulate the DC voltage and to produce either a constant voltage or constant current output as necessary for recharging the battery 180. The output of the charge controller 684, i.e., how aggressively the charge controller charges the battery 180, is dependent on the battery voltage, Vbat, as will be explained in further detail later. (The charge controller 684 can also be used to report the battery 180's charge status back to the external charge 208 via back telemetry using coil 270, as disclosed in the '263 application; however, because this function is not particularly relevant to this disclosure, it is not further discussed).
The output of the charge controller 684 is in turn met by two switches 701, 702 which respectively prevent the battery 180 from over-charging or over-discharging. As shown, these transistors are N-channel transistors, which will be “on,” and thus capable of connecting the charge controller 684's output to the battery 180 when their gates are biased. Control of these gates is provided by a battery protection circuit 686, which receives the battery current and voltage, Ibat and Vbat, as control signals, again as will be explained in further detail later. For example, whenever the battery 180 exhibits too high a voltage, the battery protection circuit 686 will turn off the gate of the over-charging transistor 701 to protect the battery from further charging. A fuse positioned between the transistors 701, 702 and the battery 180 may also be used to further protect the battery from very high current events (not shown). The battery 180 is coupled to one of several loads in the IPG 100, such as the electrode stimulation circuitry, i.e., the circuits the battery 180 ultimately powers. The battery 180 is coupled to such loads through a load switch 504, which can isolate the battery 180 from the load to protect one from adverse effects of the other. This load switch 504 is preferably part of the charge controller 684, which may comprise its own integrated circuit, although this is not strictly necessary.
As discussed in the above-referenced '263 patent, the charging circuitry 684 can charge the battery 180 in different ways, depending on the status of the battery voltage, Vbat. Without reiterating the contents of that disclosure, such selective charging of the battery 180 is beneficial for safely charging the battery, particularly when a lithium-ion-based battery is used. Essentially, this safe charging scheme charges the battery 180 with smaller currents when the battery voltage Vbat is significantly depleted, and charges with higher currents when the battery voltage is still undercharged but at higher, safer levels.
Consider an embodiment in which Vbat=4.2V represents a nominal voltage for the battery 180. When Vbat<2.5V, the charge controller 684 will “trickle” charge the battery 180 with a low level current, e.g., Ibat=10 mA. As the battery charges and as Vbat increases, higher charging current can be used. For example, once Vbat>2.5V, a charging current of Ibat=50 mA may be set by the charge controller 684. Once the nominal voltage of 4.2V is approached, the charge controller 684 may continue to charge the battery 180 by providing a constant voltage instead of constant current on its output, which as charging continues is manifest in a gradual decay of the battery current. The relationship between Vbat and Ibat during battery charging is graphically illustrated in FIG. 2. Of course, these various current and voltage values are merely exemplary, and other parameters may be suitable depending on the system at hand. Also, more than two levels of charging current (e.g., 10 mA, 25 mA, and 50 mA) can be used in stair-step fashion.
As noted earlier, the battery protection circuit 686 prevents the battery from potential damage during charging by disconnecting the battery from the charge controller 684. Specifically, Vbat exceeds a safe value (e.g., greater than 4.2V), then the over-charging transistor 701 is disabled by the battery protection circuit 686 to block further charging. Likewise, if the battery voltage is less than a predetermined value and if Ibat exceeds a predetermined value, over-discharge transistor 702 is disabled to prevent discharging of the battery. While disclosed as controlling two transistors 701, 702, the battery protection circuit 686 may control a single disabling protection transistor which functions to disable the battery 180 during both over-charging and over-discharging. Load switch 504 may be similarly controlled to isolate the components to protect them from adverse voltages and currents.
While the charging and protection circuitry of FIG. 1 is suitable, its functionality may be hampered at extremely low battery voltages. As the '263 patent explains, this is because the battery protection circuit 686 is powered by the battery voltage, Vbat, and hence when Vbat is extremely low (e.g., approaching zero Volts), the battery protection circuitry 686 may not function as desired. In this regard, note that when Vbat is extremely low, and thus when the battery 180 is in need of charging, the battery protection circuit 686 needs to be able to turn transistors 701 and 702 on, else the charging controller 684 will not be able to pass a charging current, Ibat, to the battery. However, when Ibat is low, the battery protection circuit 686 may have difficulty generating a sufficient voltage to turn on the gates of the N-channel transistors 701 and 702. Specifically, the battery protection circuitry 686 must be able to produce a gate voltage for the transistors that is greater than Vgs (i.e., the potential difference between the gate and source of the transistors). In short, the battery protection circuitry needs to be able to produce a gate voltage which exceeds a threshold voltage (Vt) of the transistors given the source voltages apparent at the transistors. If Vbat is below this threshold voltage, the battery protection circuit 686 may not be able to produce a suitably-high gate voltage to turn transistors 701 and 702 on.
Should this occur, the battery 180 cannot be charged, even though Vbat is low and hence the battery 180 is very much in need of charging. In other words, the charging and protection circuitry in FIG. 1 is potentially susceptible to failure at when Vbat is extremely low, i.e., at zero Volts or near-zero Volts. In a worst case this would mean that the IPG 100 is unrecoverable, and if implanted in a patient, may require the drastic step of surgical removal and replacement of the device. But this is unfortunate, because patients in which IPGs are implanted cannot necessarily be relied upon to diligently charge their implanted devices, and hence the risk of a depleted, unrecoverable battery is very real.
As a result, improved circuitry and techniques for protection and zero-Volt recovery for batteries in implantable medical devices would be beneficial. Such solutions are provided herein.